But how many of those people really gained coverage — that is, how many were previously uninsured?

For the first time on Thursday, the White House released data on that question. According to the Department of Health and Human Services, about 87% of people who signed up through federal health care exchanges were previously uninsured.

Here's how the percentage was determined: Of the 5.45 million people who signed up through the federal exchange, 5.18 million (or 95%) applied for financial assistance in their insurance plans. In doing so, they were required to answer a question about whether they already had health insurance. Only about 695,000 people (13%) indicated they did have coverage.

HHS said data from state-based exchanges would be available at a later date.

In total, the White House said more than 8 million people had selected insurance plans through exchanges established by the health-care law. Almost half of those — 47% — came in the last month of enrollment and during a special two-week enrollment period in April.

The administration also said more than 5 million people gained coverage through the Medicaid expansion under the Affordable Care Act. It's not known how many of those who qualified for Medicaid coverage were previously uninsured, but experts have said new enrollees in Medicaid were "primarily" without insurance before.

According to a RAND Corp. study released last month, most of the new Medicaid enrollees either did not have coverage last year or had it through other means like Medicare, retiree health insurance, and other government plans.

The White House did not say how many enrollees in new plans had paid their first premiums. On a conference call with reporters, an HHS spokesperson said it wouldn't have that data until sometime later in the year.